Pre-Christian Celts counted the passage of time in nights (starting and finishing at sunset) rather than days, with longer periods being se’nnight (seven nights) and fortnights (forten-night or fourteen night).
The concept of fortnights is much older but the word itself comes from the Old English fēowertyne niht (fourteen nights). It has long outlasted its cousin ‘sennight’ which has fallen out of use now.
Though neither word was actually used by Celts lingering in post Roman Wales (who would have had their own word for it), I’ve used it in y Ddraig so that it’s meaningful to readers (except for those in the US which is reportedly the only English-speaking country in the world not to use the term).